Optical data storage medium and use of such medium

ABSTRACT

A multi-stack optical data storage medium ( 20 ) for rewritable recording using a focused radiation beam ( 30 ), entering through an entrance face ( 25 ) is described. The medium ( 20 ) has a substrate ( 1   a   , 1   b ) and an L 0  recording stack ( 2 ) and an L 1  recording stack ( 3 ) both comprising a phase-change type L 0  and L 1  recording layer and the recording stacks are separated by a transparent spacer layer ( 4 ). The L 0  recording stack ( 2 ) is present at a position closest to the entrance face ( 25 ) and has an optical transmission of TL 0   a  and TL 0   c  when the phase-change layer respectively is in the amorphous phase and in the crystalline phase. The medium ( 20 ) contains pre-recorded information modulated in at least one of: an embossed pregroove ( 21 ) in the substrate ( 1   a   , 1   b ), embossed pits in the substrate and recorded phase-change marks in at least one of the recording layers L 0  and L 1  ( 6, 11 ). The pre-recorded information contains a flag whether formatting of the L 0  recording layer of the L 0  recording stack ( 2 ) is needed depending on the transmission values TL 0   a  and TL 0   c  of the L 0  recording stack ( 2 ). In this way it is achieved that the medium ( 20 ) has a good playability, recordability and random access behavior even when the L 0  recording layer ( 2 ) has been partially recorded with information.

The invention relates to a multi-stack optical data storage medium forrewritable recording using a focused radiation beam entering through anentrance face of the medium during recording, comprising:

-   a substrate with present on a side thereof:-   an L₀ recording stack comprising a phase-change type L₀ recording    layer, said first recording stack being present at a position    closest to the entrance face and having an optical transmission of    T_(L0a) when the phase-change layer is in the amorphous phase, and    having an optical transmission of T_(L0c) when the phase-change    layer is in the crystalline phase,-   an L₁ recording stack, comprising a phase-change type L₁ recording    layer, being present more remote from the entrance face than the L₀    recording stack,-   a transparent spacer layer between the recording stacks, said    transparent spacer layer having a thickness substantially larger    than the depth of focus of the focused laser-light beam,    the medium further comprising pre-recorded information.

The invention further relates to the use of such a medium.

An embodiment of an optical storage medium of the type mentioned in theopening paragraph is known from United States patent U.S. Pat. No.5,726,969. Rewritable optical storage for audio, video and dataapplications is a rapidly growing market. For Digital Versatile DiskReWritable (DVD+RW) the storage capacity is 4.7 Gbyte. This is a limitedamount of storage for video recording. With MPEG2 compression it ispossible to record 1 hour of high quality digital video and 2 hours ofstandard quality. More recording time is desirable. An option is to useoptical disks with multiple information layers (see FIG. 1). Such disksare already available for DVD read only (DVD-ROM). DVD rewritabledual-recording stack disks are proposed in said known patent. The L₀stack (i.e. the stack closest to the laser) has a transmission around50%. The stacks are separated by a spacer layer with a typical thicknessbetween 30 and 60 um. The L₁ stack (stack farthest from the laser) has ahigh reflection and needs to be very sensitive. The effective reflectionof both stacks is typically 7% although lower and higher values arepossible e.g. 3%–18%.

The recording layers of said stacks are of the phase-change type. Anoptical data storage medium based on the phase-change principle isattractive, because it combines the possibilities of direct overwrite(DOW) and high storage density with easy compatibility with read-onlyoptical data storage systems. Data storage, in this context, includesdigital video-, digital audio- and software-data storage. Phase-changeoptical recording involves the formation of submicrometer-sizedamorphous recording marks in a crystalline recording layer using afocused relatively high power radiation beam, e.g. a focused laser-lightbeam. During recording of information, the medium is moved with respectto the focused laser-light beam that is modulated in accordance with theinformation to be recorded. Marks are formed when the high powerlaser-light beam melts the crystalline recording layer. When thelaser-light beam is switched off and/or subsequently moved relatively tothe recording layer, quenching of the molten marks takes place in therecording layer, leaving an amorphous information mark in the exposedareas of the recording layer that remains crystalline in the unexposedareas. Erasure of written amorphous marks is realized byrecrystallization through heating with the same laser at a lower powerlevel, without melting the recording layer. The amorphous marksrepresent the data bits, which can be read, e.g. via the substrate, by arelatively low-power focused laser-light beam. Reflection differences ofthe amorphous marks with respect to the crystalline recording layerbring about a modulated laser-light beam which is subsequently convertedby a detector into a modulated photo current in accordance with therecorded information.

An important issue for multi-stack rewritable disks is random access.When a disk is recorded for the first time the L₀ recording layer of theL₀ stack, closest to the entrance face of the laser, is partiallywritten. Since the transmission of written and unwritten parts isdifferent, reading and writing of the L₁ recording layer may beinfluenced (see FIG. 2). Of special interest is the impact on the datasignal, tracking and focussing signals. It is known that an optical diskdrive can cope with relatively small differences in transmission e.g.fingerprints, dust, etc. When the transmission differences exceedcertain values the playability of the disk is strongly degraded. Insevere cases the disk will not be usable anymore. Two known solutionsare possible. In the first solution the L₀ recording layer ispre-formatted or fully written before recording of the L₁ recordinglayer. This procedure is not desirable because it is time consuming forthe user and not ideal for systems which require random access. Itshould be avoided if possible. The second solution is to optimize theoptical data storage medium such that the transmission differencebetween written and unwritten parts is very small (less than 1%). Thisis called balanced transmission for which at a 405 nm radiation beamwavelength good results have been reported by K Narumi (ISOM 2001). Forwavelengths around 650 nm it is very difficult to achieve a balancedtransmission. A disadvantage of the medium approach is that it limitsthe choice of phase-change materials which can be used. This can be adrawback for high-speed recording. The optical properties of materialssuitable for high-speed recording at wavelengths around 650 nm are suchthat balanced transmission is not feasible for realistic stacks designs.

It is likely that rewritable dual-recording stack disks will beintroduced in the market. Presently in the DVD forum there arediscussions to use dual stack disks for high-definition TV recording. Itis probable that in the near future disks with substantial transmissiondifferences between written and unwritten parts of the L₀ layer will beused. It is a problem that substantial differences in transmission willoccur in a partially written L₀ recording layer which may degrade theplayability, recordability and/or random access option of the disk.

It is an object of the invention to provide an optical data storagemedium of the type mentioned in the opening paragraph which has a goodplayability, recordability and random access behavior even when the L₀recording layer has been partially recorded with user data.

This object is achieved with the optical data storage medium accordingto the invention which is characterized in that the pre-recordedinformation contains a flag whether pre-formatting of the L₀ recordinglayer of the L₀ recording stack is needed depending on the transmissionvalues T_(L0a) and T_(L0c) of the L₀ stack.

It was found that when the optical transmissions of the user datawritten portion and the unwritten portion of the L₀ recording stack arewithin certain boundaries pre-formatting is not required. Normally, whenthe transmission difference between a written portion of user data andunwritten portion is too large pre-formatting, before recording userdata, is the only option for leveling this transmission difference. Inthe latter case, without pre-formatting, e.g. the servo signals may bedisturbed too much for successfully and randomly reading and recordingof a further portion of the recording layers of the medium. In order toselect between the option of yes/no pre-formatting, a pre-recordeddual-stack disk should contain information whether pre-formatting of theL₀ recording layer is needed or not depending on the mentionedtransmission differences. Thus pre-formatting of the L₀ recording layerbefore user data recording is an option, the necessity of it ispre-recorded in the medium by e.g. the manufacturer. The pre-recordedinformation is modulated in at least one of: an embossed pregroove inthe substrate, embossed pits in the substrate and recorded phase-changemarks in at least one of the recording layers L₀ and L₁. The option isadded as an additional flag, e.g. one or more bits, in the pre-recordedinformation using e.g. available ADdress In Pregroove (ADIP) words orother pre-recording info physically present on the disk e.g. embossedpits in case another disk format is used e.g. DVD-RW. ADIP is a methodof modulating data in the pregroove or guide track of an optical datastorage medium. The modulation is achieved by wobbling the pregroove.The information may also be pre-recorded in the recording layer of themedium e.g. the phase-change recording layer, preferably in an area,e.g. the lead-in/lead out area or disk identification area, where itdoes not or hardly disturb the recording of data by the end user. Theadvantage of the additional flag is that it remains possible to use awide range of materials with larger transmission differences. Whenapplicable, the L₀ recording layer of the medium preferably is(partially) pre-formatted or recorded prior to recording of recordinglayer L₁.

In a preferred embodiment 0.40<T_(L0a) <0.60 and 0.40<T_(L0c), <0.60.When the transmissions of the written and unwritten portions of the L₀recording stack are within these boundaries pre-formatting is notneeded. In this range of transmissions the playability, recordabilityand random access are still acceptable. Simulations and measurementsshow that the transmission of crystalline and amorphous phase changestacks may vary between 40% and 60% before degradation in the trackingand other relevant signals occurs. This makes it possible to use a wideclass of phase-change materials for the L₀ layer.

It is advantageous when a semi-transparent metal reflective layer ispresent in the L₀ stack with a thickness smaller than 15 nm. At thisthickness the metal still has a substantial optical transparency whichis required because the focused radiation beam must also reach the L₁layer. It is favorable when the metal mainly comprises one of Ag and Cu.These two metals have the additional advantage that they have arelatively large thermal conduction at a relatively low thickness whichis favorable for the optical transmission.

In another embodiment the L₀ and L₁ stacks respectively have effectiveoptical reflection values R_(L0) and R_(L1), which are substantiallyequal. This has the advantage that the modulated signals wheninformation is read back with a focused radiation beam are wellbalanced. In practice R_(L0) and R_(L1) have a value between 0.03 and0.18, typically both 0.07. The recording layers comprise at least threeelements selected from the group of the elements Ge, In, Sb and Te.Alloys of these materials have shown to be suitable for high data rateswhich requires high-speed recording. High-speed recording in its turnrequires the phase change material to have a small completecrystallization time (CET).

In a special embodiment an additional recording stack L2 is presentseparated from the L₁ stack by a transparent spacer layer having athickness larger than the depth of focus of the focused laser-lightbeam. In this case three recording stacks are present which enhances thedata capacity with approximately another 50%.

Embodiments of the optical storage medium according to the invention andmeasurement results will be elucidated in greater detail with referenceto the accompanying drawings, in which

FIG. 1 shows a schematic cross-sectional view of an embodiment of theoptical data storage medium according to the invention including an L₀-and an L1-stack. The dimensions are not drawn to scale;

FIG. 2 shows a schematic cross-sectional view of the medium of FIG. 1 inwhich the L₀ recording layer has been partially written;

FIG. 3 shows in more detail the stack design of the L₀ and the L₁ stackof an embodiment of the optical data storage medium according to theinvention;

FIGS. 4 a, 4 b and 4 c shows the HF signal taken from an optical diskdrive when reading out the LI layer when a) Lo is empty, b) L₀ is fullywritten, c) L₀ partially written

FIG. 5 shows eye patterns of data read from respectively L₀ and L₁recording layers of DL disk

FIG. 6 shows the average jitter (in %) of data read from L₁ recordinglayer of disk with either L₀ layer written after or before L₁ writing.

FIG. 7 shows the open loop push-pull-tracking signal of the L₁ recordinglayer with a partially written L₀ recording layer.

FIG. 8 shows the low frequency filtered central aperture (CA) signal and3-spots push-pull signal of an L₁ ROM layer of dual layer disk. The L₀recording layer is partially written (half-track band).

FIG. 9 shows, inter alia, Block Error Rate (BLER) data of L₁ layer ofdual layer RW/ROM disk.

FIG. 10 shows the average jitter J_(avg) (in %) and modulation M of dataread from the L₁ recording layer of a dual stack disk at different trackpositions when L₀ layer is partially written (note that L₁ is writtenafter partially writing L₀)

In FIGS. 1, 2 and 3 a schematic cross-section of a multi-stack opticaldata storage medium 20 for rewritable recording using a focusedradiation beam 30 entering through an entrance face 25 of the medium 20during recording is shown. The medium 20 comprises a substrate 1 a, 1 bwith present on a side thereof an L₀ recording stack 2 comprising aphase-change type L₀ recording layer. The L₀ recording stack 2 ispresent at a position closest to the entrance face 25 and has an opticaltransmission of T_(L0a) when the phase-change layer 6 is in theamorphous phase and of T_(L0c) when the phase-change layer 6 is in thecrystalline phase. An L₁ recording stack 3 with a phase-change type L₁recording layer 11 is present more remote from the entrance face 25 thanthe L₀ recording stack 2. A transparent spacer layer 4 is presentbetween the recording stacks and has a thickness of 52 μm, which islarger than the depth of focus of the focused laser-light beam 30. Thetransparent spacer layer 4 is made of a UV-light curable resin known inthe art and may be applied by spincoating or as a sheet of transparentplastic with pressure sensitive adhesive (PSA). The medium 20 containspre-recorded information modulated in an embossed pregroove 21 in thesubstrate by means of Address In Pregroove (ADIP). The modulation isachieved by wobbling the pre-groove 21, which is a known technique. Thepre-recorded information contains a flag whether formatting of the L₀recording layer 6 of the L₀ recording stack 2 is needed depending on thetransmission values T_(L0a) and T_(L0c) of the L₀ stack 2.

In FIG. 2 the L₀ recording stack 2 is shown which has been partiallyrecorded or written with data 2′ while the focused laser beam 30 focusesonto the L₁ recording stack 3.

In FIG. 3 two parts of a dual stack DVD+RW disk comprising asemi-transparent part with L₀ stack, called L₀, a spacer layer and apart with an L₁ stack, called L1, are shown. The L₀ part comprises apolycarbonate (PC) substrate 1 b of 0.58 mm thickness with DVD+RWpregroove structure, not drawn in this Figure, having a groove depth ofabout 30 nm and ADIP info is used. The L₀ layer stack present on thesubstrate 1 b comprises a first dielectric (ZnS)₈₀(SiO₂)₂₀ layer 5, a 6nm thick phase change layer 6, which comprises at least three elementsselected from the group of the elements Ge, In, Sb and Te, a second(ZnS)₈₀(SiO₂)₂₀ layer 7, a 3 nm Si₃N₄ capping layer 7′, a thin semitransparent metal alloy heat-sink and reflective layer 8 of 10 nm mainlycomprising Ag, a second silicon nitride capping layer 9′ and a third(ZnS)₈₀(SiO₂)₂₀ layer 9. All layers are deposited by sputtering. The L₀stack is a so-called IPII′MI′I stack, in which notation I represents adielectric layer, I′ a capping layer, P a phase-change recording layerand M a metal layer. On the substrate 1 a of the L₁ part a thick metalreflective and heat sink layer 13 being a 50 nm Al alloy, a first(ZnS)₈₀(SiO₂)₂₀ layer 12, a 12 nm phase change layer 11 and a second(ZnS)₈₀(SiO₂)₂₀ layer 10 are deposited by sputtering. The L₁ stack is anIPIM stack, in which I, P and M have the already mentioned meaning.After initialization, i.e. crystallization, of the L₀ and the L₁recording layers 6 and 11 the L₀ part and L₁ part are bonded togetherseparated by a transparent spacer 4 with a thickness between 25–60 μm,here 52 μm. The L₀ and L₁ stacks 2 and 3 respectively are designed suchthat they have effective optical reflection values R_(L0) and R_(L1)which are substantially equal and having a value of 7% each. Thetransmission T_(L0c) of the unrecorded crystalline L₀ recording stack 2is 40%, the transmission of the recorded (amorphous) L₀ stack 2 T_(L0a)is 52%. The effective transmission of a recorded part is close to 43%,i.e. ¼ of area is mark. Thus for the transmission of the written area ofthe L₀ stack 2 a transmission equal to (3T_(L0c)+T_(L0a))/4 was assumed.An additional recording stack L2, not shown, may be present separatedfrom the L₁ stack by a transparent spacer layer having a thicknesssubstantially larger than the depth of focus of the focused laser-lightbeam. In this case the transmission levels of the L₀ and L₁ stacks haveto be adjusted in order to balance effective reflection values form theL₀, L₁ and L2 stacks.

In FIG. 4 the HF read out signal of the L₁ recording layer 6 from a dualstack disk is shown for three cases. In 4 a the trace with L₀ unwrittenis shown, in 4 b with L₀ written and in 4 c for a partially written L₀layer, i.e. bands of 70 pregroove tracks written/unwritten. Clearly inthe HF signal some effect of the written tracks in the L₀ layer can beseen. Recording results in the form of eye patterns are shown in FIG. 5for the L₀ and L₁ recording layers.

In FIG. 6 Direct Overwrite (10 DOW cycles) average jitter J_(avg) isaround 11% for the L₁ recording layer 6, which is represented by graphs61 to 64. J_(avg) levels of the L₁ recording layer 11 of below 9% mayalso be achieved, which is shown in FIG. 10. Jitter of the L₀ recordinglayer is around 9%. The average jitter value of the L₁ recording layer 2is slightly affected by the status of L₀ recording layer 2. Twoexperiments are shown. In the first experiment the L₁ recording layer iswritten first and afterwards L₀, in this way the read effects arestudied because L₁ writing is always the same independently from the L₀pattern. Graph 61 represents a linear fit of jitter data before L₀ iswritten and 62 after L₀ is written. In the second experiment L₀ iswritten first and afterwards L1, in this way both the influence of apartially written L₀ disk on the writing and the reading is simulated.Graph 63 represents a linear fit of jitter data before L₀ is written and64 after L₀ is written. The latter method (L₀ first) is also used inFIG. 10.

FIG. 7 shows the open loop push-pull-tracking signal of the L₁ recordinglayer with partially written L₀ recording layer. The open loop signal isthe tracking signal when the laser is not tracking on a pregroove, i.e.the tracking servo is not activated. No substantial disturbance isvisible in the signal.

In FIG. 8 it is shown that normalized push pull and central aperture(CA) signals 82 and 81 were not severely degraded. In order to test theinfluence of a partially written L₀ recording layer on the bit errorrate data rate of the L₁ layer a dual layer disk with a read only ROM L₁layer was fabricated. The ROM disk data consisted of a video stream (ECCcorrect data). The reflection of the ROM disk was comparable with thatof the previous L₁ layer. As a test pattern a band of half-tracks waswritten in L₀, in which case only a 180-degree portion of the fullrotation of several adjacent tracks is recorded. This is considered asthe worst case pattern for bit errors. The 3-spot push pull signal 82 ofthe ROM layer is shown where the focused laser beam crosses an L₀written/unwritten transition. The normalized signal is affectedslightly, but no substantial offset can be observed. Tracking is withoutproblem but note that the push-pull signal 82 is small because of theuse of the ROM disk as L₁ disk. The block error rate (BLER) was low, allblocks could be corrected and no effect of the partially written L₀recording layer is observed even for this worst case situation, which isshown in FIG. 9 in curve 91. Other measured parameters in FIG. 9 are notconsidered relevant at this time.

The impact of a partially written L₀ recording layer 6 was investigatedby modeling of the push-pull signal. The calculation results are shownin Table 1 as a function of the transmission T_(L0a) and T_(L0c). Theposition of the focused laser beam 30 is such that half of the beampasses a written area. For the transmission of the written area atransmission equal to (3T_(c)+T_(a))/4 was again assumed. Clearly for0.40<T_(L0a)<0.60 and 0.40<T_(L0c)<0.60 the offset in the push pullsignal is small (<2 nm) and significantly smaller than the allowed 16 nmwhich is the maximum allowed value according to the DVR+RW formatspecification book version 1.1.

Hence both experimental results and simulations show that for arealistic range of transmission values T_(L0a) and T_(L0c) of 0.40 to0.60 the influence of writing information in the L₀ recording layer 6 onthe tracking and read-out system is substantially absent

TABLE 1 Calculated push-pull offset (in nm) for variable transmissiondifferences of written and empty tracks. The position of the focusedlaser beam is such that half of the beam 30 passes a written area.T_(L0c) T_(L0a) 0.4 0.5 0.6 0.4  0.0 nm 0.78 nm 1.33 nm 0.5 −0.93 nm  0.0 nm 0.65 nm 0.6 −1.8 nm −0.75 nm   0.0 nm

In FIG. 10 the influence of a non-written or written L₀ recording layer6, when the L₁ recording layer 11 is written through the partiallywritten L₀ recording layer 6, on the average jitter J_(avg) of an L₁recording layer of DVD+RW Dual Stack disk is shown. At positions wherethe L₀ recording layer has been written the J_(avg) decreases about0.5%. Variation in J_(avg) along the disk is about 0.5%. The patternwritten in the L0 recording layer consisted of 5 times 100 tracksEFM+alternated with 100 empty (non-written) tracks, 500 tracks L₀ EFMdata (a half-circular pattern is written as described with FIG. 8), 100empty tracks and 1000 tracks of EFM+L₀. This pattern is clearly visiblein the modulation (M) curve 102. In areas where the L₀ recording layeris written the J_(avg) decreases because the transmission of the L₀stack is about higher, i.e. about 10% relatively. This causes anincreased laser recording power in the L₁ recording layer which in itsturn causes the modulation to increase and the J_(avg) to drop, which isclearly visible in curve 101.

It should be noted that the above-mentioned embodiments and experimentaldata illustrate rather than limit the invention, and that those skilledin the art will be able to design many alternative embodiments withoutdeparting from the scope of the appended claims. In the claims, anyreference signs placed between parentheses shall not be construed aslimiting the claim. The word “comprising” does not exclude the presenceof elements or steps other than those listed in a claim. The word “a” or“an” preceding an element does not exclude the presence of a pluralityof such elements. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measures cannot be used to advantage.

According to the invention, a multi-stack optical data storage mediumfor rewritable recording using a focused radiation beam, enteringthrough an entrance face is described. The medium has a substrate and anL₀ recording stack and an L₁ recording stack both comprising aphase-change type L₀ and L₁ recording layer and the recording stacks areseparated by a transparent spacer layer. The L₀ recording stack ispresent at a position closest to the entrance face and has an opticaltransmission of T_(L0a) and T_(L0c) when the phase-change layerrespectively is in the amorphous phase and in the crystalline phase. Themedium contains pre-recorded information modulated in at least one of:an embossed pregroove in the substrate, embossed pits in the substrateand recorded phase-change marks in at least one of the recording layersL₀ and L₁. The pre-recorded information contains a flag whetherformatting of the L₀ recording layer of the L₀ recording stack is neededdepending on the transmission values T_(L0a) and T_(L0c) of the L₀recording stack. In this way it is achieved that the medium has a goodplayability, recordability and random access behavior even when the L₀recording layer has been partially recorded with information.

1. A multi-stack optical data storage medium for rewritable recordingusing a focused radiation beam entering through an entrance face of themedium during recording, comprising: a substrate with present on a sidethereof: an L₀ recording stack comprising a phase-change type L₀recording layer, said first recording stack being present at a positionclosest to the entrance face and having an optical transmission ofT_(L0a) when the phase-change layer is in the amorphous phase, andhaving an optical transmission of T_(L0c) when the phase-change layer isin the crystalline phase, an L₁ recording stack, comprising aphase-change type L₁ recording layer, being present more remote from theentrance face than the L₀ recording stack, a transparent spacer layerbetween the recording stacks, said transparent spacer layer having athickness substantially larger than the depth of focus of the focusedlaser-light beam, the medium further comprising pre-recordedinformation, characterized in that the pre-recorded information containsa flag whether formatting of the L₀ recording layer of the L₀ recordingstack is needed depending on the transmission values T_(L0a) and T_(L0c)Of the L₀ stack.
 2. An optical data storage medium as claimed in claim1, wherein the pre-recorded information is modulated in at least one of:an embossed pregroove in the substrate, embossed pits in the substrateand recorded phase-change marks in at least one of the recording layersL₀ and L₁.
 3. An optical data storage medium as claimed in claim 1,wherein 0.40<T_(L0a)<0.60 and 0.40<T_(L0c)<0.60.
 4. An optical datastorage medium as claimed in claim 1, wherein a semi-transparent metalreflective layer is present in the L₀ stack with a thickness smallerthan 15 nm.
 5. An optical data storage medium as claimed in claim 4,wherein the metal mainly comprises one of the elements Ag and Cu.
 6. Anoptical data storage medium as claimed in claim 1, wherein the L₀ and L₁stacks respectively have effective optical reflection values R_(L0) andR_(L1) which are substantially equal.
 7. An optical data storage mediumas claimed in claim 6, wherein R_(L0) and R_(L1) have a value between0.03 and 0.18.
 8. An optical data storage medium as claimed in claim 1,wherein the recording layers comprise at least three elements selectedfrom the group of the elements Ge, In, Sb and Te.
 9. An optical datastorage medium as claimed in claim 1, wherein an additional recordingstack L2 is present separated from the L₁ stack by a transparent spacerlayer having a thickness larger than the depth of focus of the focusedlaser-light beam.
 10. Use of an optical data storage medium according toclaim 1 for multi-stack and random access rewritable data recording.